1. Field of the Invention
The present invention relates to positioning devices and more particularly to a positioning device which is used to provide controlled, accurate positioning of a movable member.
2. Description of the Prior Art
Positioning devices currently utilized in positioning a member such as a tool or cutter typically include a slide, sometimes moving with a ball bearing arrangement, and a leadscrew which provides the driving force and positioning. These slides are subject to three problems which prohibit discrete movements of less than 50 millionths of an inch. These three problems are "stick-slip", "backlash" and wear of bearing surfaces.
"Stick-slip" is the tendency of the moving member of a slide to stick or freeze when stationary, due to friction between the adjacent surfaces. Although this problem is initially overcome by a ball bearing slide, wear and contamination eventually produce "stick-slip". The only recourse to this problem is to infeed an additional amount until the slide moves or to tap the slide with a hammer until it moves. When using the first method, the resulting movement is usually a lunge or jump which can be two or three times the desired movement. The tapping of the slide with a hammer, although crude, can be effective but cannot be consistently relied upon for extremely accurate positioning.
"Backlash" is the tendency of leadscrews to develop a free movement before exerting any pressure on the slide member. This is particularly true where a reverse movement is desired, such as when the member is advanced too far and a small retraction is needed. Theoretically, "backlash" occurs since clearance is required between a screw and a nut to prevent it from locking up. However, when the screw is reversed, it uses the back side of the thread and develops a "play" or "backlash" when traversing from the front side of the thread to the back side of the thread through the clearance provided. This "backlash" may seem insignificant but does prevent precise, minute incremental positioning which is frequently needed.
The third major problem with current positioning devices is the wear of the surfaces of the slide. As stated above, initial results are satisfactory, especially with a ball bearing slide. However, friction between the surfaces produces wear which eventually leads to play in the slide and an inevitable inaccurate positioning. This problem is enhanced by contamination from foreign matter which is produced by most machining operations.
Additionally, some machines must provide a means of firmly locking the slide at a given point. This is accomplished by different means but usually results in a deflection of the slide sufficient enough to cause inaccurate positioning.
Heretofore various methods and apparatus have been proposed for providing accurate positioning of a movable member. Examples of such methods and apparatus can be found in the following U.S. Patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 1,122,713 Fisher 1,322,352 Schellenbach 2,065,966 Bruning 2,370,742 Mercer 2,377,239 Kasen 2,391,142 Drake 2,852,966 Olson 3,361,018 Druckman ______________________________________
The Fisher U.S. Pat. No. 1,122,713 discloses a tool adjustment for lathes. This patent discloses a slide which moves a cutting tool and has weights attached to the slide to provide an axial load on the slide which is driven by a screw feed.
The Schellenback U.S. Pat. No. 1,322,352 discloses an automatic lathe using a slide which moves a cutting tool and is driven by a screw feed.
The Bruning U.S. Pat. No. 2,065,966 discloses an elastic holder for cutting tools. This patent discloses a tool holder in which the tool can give way elastically if the tool or workpiece are in danger of being damaged or broken. The elastic member is constituted by a plate spring or by a plurality of superimposed plate springs.
The Mercer U.S. Pat. No. 2,370,742 discloses another tool holder which is capable of yielding to avoid distortion or breakage and also uses plate springs.
The Kasen U.S. Pat. No. 2,377,239 discloses a set-up and adjustment means whereby a cutting tool is moved on a slide which is driven by a cam capable of being adjusted by a screw. The slide is additionally subjected to an axial force provided by helical springs.
The Drake U.S. Pat. No. 2,391,142 discloses a tool holder which will yield to permit the tool to move from the workpiece. This yielding means is provided by a helical spring having a spring rate which may be varied to provide different tensions to control the action of the tool.
The Olson U.S. Pat. No. 2,852,966 provides a cutting tool holder which will retract from the cutting surface without marring and is capable of minute adjustments. This patent discloses a tool holder which moves along a slide subjected to a force provided by a helical spring.
The Druckman U.S. Pat. No. 3,361,018 discloses a tool holder for lathes in the form of a pair of flat parallel leaf springs having their lower edges secured to the lathe and their upper ends mounting a tool holder.
As will be described in greater detail hereinafter, the apparatus of the present invention provides a positioning device which is capable of repeated accurate positioning of a member within five millionths of an inch. This accuracy has never been achieved before and is provided in an inexpensive, dependable and more rugged apparatus which is not susceptible to the problems described above and which will continue to function in the most hostile manufacturing environments with a minimal amount of maintenance.
Moreover, the apparatus of the present invention differs from the previously proposed apparatus by providing an incremental positioning device which is not subjected to friction between surfaces, does not exhibit any free movement before positioning is started or reversed, and moves in an absolute straight line.